This is a question I've been meaning to ask for a long time.

I sometimes use pages 104-5 of the Simandl book as practice. But realistically how much of the Simandl book could I also use to practice?

I guess you couldn't really use anything that doesn't deal with specific legato bowings on BG, but the rest of the first book has some nice exercises. DB's have a higher range than BG's though, so you'll eventually reach a point, even on a 24 fret neck, where some of the stuff in the second book would be physically impossible.

Realistically, all of it could be helpful. The fingerings up starting in the fourth position (seventh fret or position on electric) might not be very helpful, since that's definitely a good OFPF territory, but following the fingerings in the lower positions is a good idea. If nothing else, its good reading practice, and will help you work your way around the fretboard.

Page 104-5 is a really good workout, btw, I'm glad you mentioned that. 83-85 is another good one (exercise in all major/minor keys), and if you're working on developing a nice, legato touch (Jeff Berlin style), 101 is a good page to work on too.

You won't get into the ridiculously high stuff until book 2. In fact, book one stops before thumb position, so you'll never play anything higher than the 14th fret. There are some etudes written with harmonics in it that you'd need a 25 fret 9-string bass with a high Bb string to play. Yeah, it's pretty ridiculous. That book is also a really good one to get because it covers the whole of the fingerboard, and so studying it will be good for your ability to play in the upper register.

When I was in college Simandl's book was a requirement for everyone in the bass department, regardless of whether you played upright or electric. The upright guys just had to pay more attention to the fingering & bowing instructions; us electric guys basically used it as a reading exercise.

All of Simandl can be used in one way or another and something gained. I would add the Rufus Reid book too.

Any music book can help over in Janek Gwizdala's forum awhile back it was discussed using the Hanon piano book for bass practice. I have trombone books and many jazz books are written for any instrument. DB and other traditional instruments their method books have gone thru hundreds of years of refinement on how to teach the fundamentals of the instrument and music. I can hear some grumbling out there we're new and cutting edge. Well you can still learn a lot from traditional methods. Try slapping some Simandl, four finger technique of the chord studies, play it faster/slower, change the rhythm. Some exercises may give you some new ideas for counterpoint behind your blasting guitarists. Use your creativity, that make those same twelve notes people have been playing for hundreds of years sound old or new.